The present invention relates to methods and apparatus for producing power and hydrogen (H2) from a gaseous mixture comprising H2 and carbon dioxide (CO2), and in particular for concurrently and/or adjustably producing electric power and a high purity hydrogen product (preferably having a purity of at least about 99.9 mole %, more preferably at least about 99.99 mole %) from a gaseous mixture obtained from gasification of or from reforming a carbonaceous feedstock.
Gasification of a solid or liquid carbonaceous feedstock, or partial oxidation or steam methane reforming of a gaseous or liquid carbonaceous feedstock, followed by subsequent separation of hydrogen from the gasifier or reformer effluent, is a well known technique of producing hydrogen, and has been a topic of research and development for many years. As is also known, the separated hydrogen product may then be put to a number of uses, depending on its purity. For example, hydrogen may be used as a fuel in for example a gas turbine, thereby generating power (in particular electric power), and/or it may be used in refinery, chemicals and/or fuel cell applications. Where the hydrogen product is to be used as a fuel for a gas turbine for generating power, a somewhat lower purity is typically acceptable than that which is required where the hydrogen product is intended for refinery, chemicals or fuel cell applications (all of which typically require an H2 purity of at least 99.9 mole %, and more typically at least 99.99 mole %).
Gasifier or reformer effluent typically comprises H2, CO2 and carbon monoxide (CO) as the major components, with minor amounts of other components such as methane (CH4), ammonia (NH3), nitrogen (N2), argon (Ar) and, where the feedstock contained sulphur, certain sulphur containing species (predominantly hydrogen sulphide (H2S), but other species such as carbonyl sulphide (COS) and carbon disulphide (CS2) may to a lesser extent also be present). This effluent is often then subjected to a water-gas-shift reaction to convert, by reaction with H2O, some or all of the CO to CO2 and H2. In circumstances where any sulphur containing species are not first removed by appropriate sorptive techniques (as may be necessary where a sulphur sensitive shift catalyst is to be used) this can have the side-effect of also increasing the concentration of H2S in the shifted mixture, due to conversion of other sulphur species in the crude syngas stream to H2S during the water-gas-shift reaction.
If an H2 product suitable for use as a fuel for generating power or for use in refinery, chemicals or fuel cell applications is desired, further separation of the H2 from the other components of the gasifier, reformer or shift-converter effluent will typically then be required. An array of technologies for the separation of H2 from such mixtures, and from other mixtures comprising H2 and CO2, have been developed and are known. One approach is to utilize pressure swing adsorption (PSA), and a variety of methods adopting this approach have been described in the art.
For example, US-A1-2007/0178035 describes a method of treating a gaseous mixture, such as obtained from a gasification process, comprising H2, CO2 and one or more combustible gases (i.e. H2S, CO and CH4). H2 is separated, preferably by pressure swing adsorption (PSA), from the gaseous mixture to produce a separated high purity H2 gas and a crude CO2 gas comprising the combustible gases. The crude CO2 gas is combusted to produce heat and a CO2 product gas comprising the combustion products of the combustible gas(es). Heat is recovered from the CO2 product gas by indirect heat exchange with the H2 gas, to which a diluent (e.g. N2 or H2O) may have been added, and the warmed H2-containing gas may then be fed as fuel to a gas turbine. Where the combustion product(s) comprise SOx (SO2 and SO3), these may be removed by a process that involves washing the gas with water and maintaining the gas at elevated pressure.
U.S. Pat. No. 4,171,206 describes a method in which two PSA systems, each comprising a plurality of adsorbent beds operating in parallel, are used in series to separate a high purity H2 product and a CO2 product from a feed gas comprising H2 and CO2 and one or more dilute components, such as CO and CH4. The feed gas may for example be produced from a shift converter in a hydrocarbon reforming plant. The feed gas is fed to the first PSA system at super-atmospheric pressure, and CO2 is adsorbed. The unadsorbed gas pushed through the first PSA system is then fed to the second PSA system where the dilute components are adsorbed, and the unadsorbed gas pushed through the second PSA system is withdrawn as high purity H2 product. The first PSA system employs a vacuum pressure swing adsorption process, whereby the desorbed gas obtained at ambient and sub-ambient pressures during blowdown and evacuation of the beds of the first PSA system is withdrawn as high purity CO2 product. The desorbed gas obtained at about ambient pressure during blowdown/purging of the beds of the second PSA system is withdrawn as a product containing H2, CO and CH4 and having good fuel value.
U.S. Pat. No. 4,790,858, U.S. Pat. No. 4,813,980, U.S. Pat. No. 4,836,833, U.S. Pat. No. 5,133,785 describe a number of modifications to or variations on the method described in U.S. Pat. No. 4,171,206. U.S. Pat. No. 4,790,858 describes a method in which the product containing H2, CO and CH4 obtained at atmospheric pressure from the second PSA system is compressed and fed to a third PSA system, so as to recover some of the H2 present in said feed as further high purity H2 product. U.S. Pat. No. 4,813,980 describes the use of first and second PSA systems to separate a reformer off-gas, comprising H2, N2, CO2 and minor quantities of CH4, CO and Ar, into a high purity ammonia synthesis gas (e.g. a product comprising a 3:1 ratio of H2 to N2), a high purity CO2 product, and a product containing H2, CH4 and CO that can be used as fuel for the reformers. U.S. Pat. No. 4,836,833 describes a method in which the feed to the first PSA system is the reformate from a steam methane reformer, and the desorbed product obtained from the second PSA system contains CO, H2 and minor amounts of CH4 and is further separated in a multi-membrane system to obtain a high purity CO product. U.S. Pat. No. 5,133,785 describes certain modifications to the PSA cycle described in U.S. Pat. No. 4,171,206 for operation of the first and second PSA systems.
U.S. Pat. No. 3,102,013 discloses a method of separating a mixture of at least three components, designated A, B and C, using at least two PSA beds in series. The mixture is fed to the first bed at high pressure, where component C is adsorbed, and the unadsorbed gas pushed through the first bed is fed to the second bed, where component B is selectively adsorbed, thereby obtaining a product comprising component A. A portion of this product is used to purge the beds at low pressure. The gas purged from the first bed comprises components A and C and the gas purged from the second bed comprises components A and B. These purged gases are then separated in further separation beds into components A and C and A and B, respectively.
U.S. Pat. No. 4,042,349 discloses methods of separating mixtures using two or more PSA beds in series and/or in parallel. In one embodiment two beds are used in series, and in parallel with two further beds in series, to separate an H2 stream from a feed mixture comprising H2, N2, CH4, Ar and NH3.
U.S. Pat. No. 4,539,020 discloses a method of separating CO from a feed gas comprising CO2, CO and a less adsorbable component than CO, such as N2, H2 or CH4, through PSA using in series at least two adsorbent beds. The first bed selectively adsorbs CO2 from the feed gas, and the CO2 depleted gas pushed through the first bed is fed to the second bed which selectively adsorbs CO. The gas pushed through the second bed comprises CO and the less adsorbable components and can be used for purging the first bed, with the remainder being usable as a fuel in view of its considerable CO content. The gas evacuated from the second bed under vacuum forms the high purity CO product. In one example, the process is used to separate a gaseous mixture comprising CO, CO2, N2, H2 and O2 which is an off-gas from a converter furnace.
U.S. Pat. No. 4,696,680 describes a method for bulk separation of a gaseous mixture, comprising predominantly H2, CO, CH4, CO2 and H2S, derived from the gasification of coal. In one embodiment, the gaseous mixture is fed at about atmospheric pressure to a first PSA bed which selectively adsorbs CO2 and H2S. The non-adsorbed gas, which comprises H2, CO and CH4, from the first PSA bed is compressed and fed to a second PSA bed at a pressure at which H2, CO and CH4 are all adsorbed. The pressure in the second PSA bed is then gradually decreased to sequentially desorb a high purity H2 product, a CO enriched product and a CH4 enriched product. The first PSA bed is regenerated by desorbing the CO2 and H2S at sub-atmospheric pressure. The CO and CH4 enriched products may be utilized as a mixture for providing fuel gas.
U.S. Pat. No. 4,761,167 describes a method of removing N2 from a fuel gas stream comprising CH4, N2 and CO2. The fuel gas stream is fed to a PSA system, comprising a plurality of adsorbent beds employed in parallel that selectively adsorb CO2 from a mixture. The unsorbed effluent, consisting substantially of CH4 and N2, exiting the PSA system is then fed to a Nitrogen Rejection Unit (NRU) that separates the N2 from the CH4 by fractional distillation. The nitrogen stream obtained from the NRU can then be used for purging the beds of the PSA system during regeneration of the beds at atmospheric pressure.
U.S. Pat. No. 6,340,382 describes the design and operation of a PSA system for producing a high purity (≧99.9%) H2 product from a gas stream containing more than about 50 mole % H2, such as streams that contain from 60 to 90 mole % H2 and include CO2, H2O, CH4, N2 and CO. The document also cross-references a number of previous works on PSA cycles and adsorbent options for producing high purity H2.
US2007/0199446 describes a vacuum pressure swing adsorption (VPSA) process for producing an essentially CO-free hydrogen gas stream from a high-purity, e.g. pipeline grade, hydrogen gas stream using one or two adsorber beds. The high-purity hydrogen gas stream consists of about 99.9% by volume H2 with up to about 1000 ppm of non-hydrogen impurities, and the essentially CO-free hydrogen gas stream contains less than 1 ppm CO. The PSA process uses physical adsorbents with high heats of nitrogen adsorption, intermediate heats of carbon monoxide adsorption, and low heats of hydrogen adsorption, and uses vacuum purging, high feed stream pressures (e.g. feed pressures of as high as around 1,000 bar (100 MPa)) and feed times of greater than around 30 minutes to produce the essentially CO-free hydrogen from the pipeline grade hydrogen.
US-A1-2007/0227353 describes a method of separating a CO2 product having a purity of at least 80 mole % from a feed stream containing at least CO2 and H2 via VPSA. The feed may for example be a syngas stream, obtained from steam methane reforming and shift-converting natural gas, which is fed to the VPSA unit at super-atmospheric pressure. The H2-enriched unsorbed effluent is sent to a second PSA unit where it is further separated to obtain high-pressure, high purity H2 product. The gas desorbed from the VPSA unit at sub-atmospheric pressure is withdrawn as the CO2 product, and the gas desorbed from the second PSA unit may be used as a fuel stream for the steam methane reformer.
U.S. Pat. No. 7,550,030 and US-A1-2008/0072752 describe variations on the method described in US-A1-2007/0227353. In the method of U.S. Pat. No. 7,550,030, a third stream is obtained from the VPSA unit, which stream is an H2-depleted stream (relative to the feed to the VPSA unit) which is formed from gas desorbed from the beds of the VPSA during depressurization of the beds prior to evacuation of the beds at sub-atmospheric pressure. This H2-depleted stream may then be mixed with gas desorbed from the second PSA unit, to form a combined fuel stream for the steam methane reformer, or may be sent to an incinerator or vented. In the method of US-A1-2008/0072752, a stream formed from gas desorbed from the beds of the VPSA unit during depressurization of the beds prior to evacuation of the beds at sub-atmospheric pressure is recycled into the fresh feed to the VPSA unit.
WO2005/118126 describes a method of producing high purity hydrogen, in which a hydrocarbon feed is reformed at high pressure in a partial-oxidation or steam-methane reformer to produce a high pressure effluent containing H2 which is separated in a PSA unit to produce a high purity product stream (i.e. 98 volume % H2 or higher). The H2 containing gas purged from the PSA unit may be combusted to heat the feed air to the reformer. Where the hydrocarbon feed is a sour feed (i.e. contains H2S), an H2S sorber, containing for example a sorbent such as zinc oxide, may be used to remove H2S from the reformer effluent prior to separation in the PSA unit.
FR2899890 describes a PSA process for producing a H2 product (98-99.5 mole % purity) from a feed gas containing hydrogen, in which the gas used to purge the beds of the PSA unit during the purge step of the PSA process is an H2 rich gas which is at least partly obtained from an external source, such as from a petrochemical or oil unit in an oil refinery.
It is an objective of preferred embodiments of the present invention to provide efficient and flexible production of both power and hydrogen from a gaseous mixture comprising H2 and CO2, such as for example a mixture obtained from gasification of or reforming hydrocarbon feedstock.
Operation of a plant to make both a high purity H2, for example for selling to a customer, and a lower purity H2 stream for use as a fuel for making power by combustion in, for example, a gas turbine, can be desirable for a number of reasons. In particular, having the capability to make both electric power and high purity H2 has the potential for significant cost advantages. Due to economies of scale, the incremental capital and operating cost of making power alongside high purity H2 is potentially significantly less than that for making the same amount of power and/or high purity H2 in standalone plants.
There can also be advantages in having the flexibility to vary production between a high purity H2 for sale and a lower purity H2 for use as a fuel for making power. For example, the price of electric power can vary considerably, with peaks and troughs in demand depending upon factors such as the time of the day or the season. There could therefore be commercial benefit in being able to turn down or turn off gas turbines when the price of electric power is low and ramp up the production of high purity H2 when it can be sold at a higher price than power. Likewise, when the price of electric power is high it could be commercially beneficial to be able reduce or halt production of high purity H2 in order to increase production of electric power.
In addition, there may be circumstances in which the source of the gaseous mixture (from which both power and H2 are to be produced) cannot be completely relied upon. For example, in circumstances where the gaseous mixture is obtained from gasification of a carbonaceous feedstock by several gasifiers, it may be that one or more gasifiers, which are known to be somewhat unreliable, suddenly and unexpectedly fail during normal operation. Where the plant ordinarily produces both power and high purity H2 and has the ability to vary production of the same, the plant operator may at least have the option of reducing or ceasing production of power or high purity H2 in order that desired levels of production of the other are maintained. For example, where high purity H2 is required for continuous supply to a customer, the ability to maintain the level of supply to the customer by, if necessary, reducing or halting (at least temporarily) power production can provide the customer with a more reliable service.